As is known in the art, roofs can be rated according to UL 790 or ASTM E 108 standards for fire resistance. A Class A roof has the highest resistance to fire. An unrated roof has the lowest resistance to fire, and class B and class C rated roofs are rated therebetween, respectively. These ratings are generally based upon flame penetration into the attic space through the roof covering, flame spread over the surface of the roof covering, and the propensity for the roof covering to become dislodged and generate embers.
Certain non-combustible roof decks can achieve a Class A rating by virtue of their construction. For example, a concrete deck typically achieves a Class A rating. Other roofs, such as those carrying an aluminum covering, may require complementary materials to achieve a Class A rating. For example, an asphaltic underlayment may be placed under an aluminum roof covering to achieve a Class A rating.
Where the roof deck is a combustible deck, such as a wood deck, it can be extremely difficult to achieve a Class A rating. In many situations, robust and expensive underlayment is required to achieve a Class A rating over a combustible deck. For example, fiberglass-backed gypsum board (e.g., DensDeck™) can be applied over a combustible deck to achieve a Class A rating. Alternatively, or in addition thereto, a heavy underlayment (e.g., 72 pound felt or cap sheet) may be required to achieve a Class A rating over a wood roof deck.
The difficulty in achieving a Class A rating can be further complicated by the type of roof covering employed over the combustible deck. Where a roof covering is metal (e.g., aluminum), the complementary construction material, such as an underlayment, may not need to be as robust to achieve a Class A rating. On the other hand, where the roof covering is a polymeric membrane (e.g., a thermoplastic sheet), the ability to achieve a Class A rating over a combustible deck is not trivial.